To solve this problem, we need to determine during which phase the centriole undergoes duplication.$\newline$Let's analyze the cell cycle phases:$\newline \bullet$G$_1$-phase: This is the first gap phase where the cell grows and prepares for DNA replication. Centriole duplication does not occur here.$\newline \bullet$S-phase: This is the synthesis phase where DNA replication occurs. Importantly, centriole duplication also takes place during this phase.$\newline \bullet$